Conformational stability of Cu,Zn-superoxide dismutase, the apoprotein, and its zinc-substituted derivatives: second-derivative spectroscopy of phenylalanine and tyrosine residues

The relative stabilities of bovine copper-zinc superoxide dismutase (SOD), its apoprotein form, and zinc-substituted derivatives were investigated by denaturation in guanidine-HCl solutions. Analysis of the kinetics of changes in the second-derivative spectral bands of both phenylalanine and tyrosine residues was simultaneously performed. It was found that reduction of the cupric site increases the stability of the enzyme. The apoprotein appears to be the least stable form, while addition of zinc ions not only increases stability, but appears to induce a native-like conformation from a disordered form at pH 3.8. By perturbing the solvent with up to 20% ethylene glycol, at pH 6.8, it was determined that the only tyrosyl side chain appears to be about 50% solvent-exposed in the apoprotein, 65% exposed in the zinc derivative, and 75% exposed in the native copper-zinc form. In contrast, all four phenylalanine residues appear to be fully buried in all of these species in the mid-pH range. At pH 2.5, as the apoprotein unfolds, the apparent solvent-exposure of the tyrosyl side chain approaches 100%, while the phenylalanyl side chains become only 70% exposed. Substantial differences in the unfolding rate constants of tyrosine and phenylalanine residues of native and zinc-substituted SOD, but not the apoprotein, suggest the presence of metal-stabilized unfolding intermediates. Unfolding as monitored by the exposure of phenylalanine residues follows first-order kinetics, indicating that Phe 48 located at the interface between the two subunits is being exposed to the solvent simultaneously with the remaining three phenylalanine residues buried in the protein core.     

Key sites residues involved in interacting with chemicals of pheromone‐binding proteins from Lymantria dispar

Pheromone‐binding proteins (PBPs) are distributed widely on the antennae of insects, and they are believed to be involved in the process of chemical signal transduction, but their interaction with chemicals is largely unknown. Here, we present our findings on the key amino acid residues of PBPs in the gypsy moth, Lymantria dispar. Potential key residues were screened with the Calculate Mutation Energy program and molecular docking methods. Mutated proteins were obtained by mutating residues to alanine via site‐directed mutagenesis. Circular dichroism (CD) spectroscopy showed that the mutated proteins formed α‐helix, and the stability of protein structure was influenced due to mutations. Fluorescence binding assays were further conducted with the mutated proteins, sex pheromones and analogues. Results showed that to PBP 1, tyrosine at position 41 and phenylalanine at position 76 could be the key amino acid residues influencing the stability of structure; in addition, phenylalanine at 36 and lysine at position 94 could be key amino acid residues interacting with chemicals. To PBP 2, glycine at position 49, phenylalanine at position 76 and lysine at position 121 could be the key amino acid residues in the structural stability. These results shed light on the relationship between the specific amino acids and functions of PBPs in transmitting the chemical signals.     

An ultraviolet photoacoustic spectroscopy study of the interaction between Lys49-phospholipase A2 and amphiphilic molecules

We have used near ultraviolet photoacoustic spectroscopy (PAS) over the wavelength range 240-320 nm to investigate the complex formed between the homodimeric bothropstoxin-I, a lysine-49-phospholipase A2 from the venom of Bothrops jararacussu (BthTx-I), with the anionic amphiphile sodium dodecyl sulfate (SDS). At molar ratios>10, the complex developed a significant light scatter, accompanied by a decrease in the intrinsic tryptophan fluorescence intensity emission (ITFE) of the protein, and an increase in the near UV-PAS signal. Difference PAS spectroscopy at SDS/BthTx-I ratios<8 were limited to the region 280-290 nm, suggesting initial SDS binding to the tryptophan 77 located at the dimer interface. At SDS/BthTx-I ratios>10, the intensity between 260 and 320 nm increases demonstrating that the more widespread tyrosine and phenylalanine residues contribute to the SDS/BthTx-I interaction. PAS signal phase changes at wavelengths specific for each aromatic residue suggest that the Trp77 becomes more buried on SDS binding, and that protein structural changes and dehydration may alter the microenvironments of Tyr and Phe residues. These results demonstrate the potential of near UV-PAS for the investigation of membrane proteins/detergent complexes in which light scatter is significant.     

Temperature dependent 2nd derivative absorbance spectroscopy of aromatic amino acids as a probe of protein dynamics

Proteins display a broad peak in 250–300 nm region of their UV spectrum containing multiple overlapping bands arising from the aromatic rings of phenylalanine, tyrosine, and tryptophan residues. Employing high resolution 2^nd derivative absorbance spectroscopy, these overlapping absorption bands can be highly resolved and therefore provide a very sensitive measure of changes in the local microenvironment of the aromatic side chains. This has traditionally been used to detect both subtle and dramatic (i.e., unfolding) conformational alterations of proteins. Herein, we show that plots of the temperature dependent 2^nd derivative peak positions of aromatic residues have measurable slopes before protein unfolding and that these slopes are sensitive to the dielectric properties of the surrounding microenvironment. We further demonstrate that these slopes correlate with hydration of the buried aromatic residues in protein cores and can therefore be used as qualitative probes of protein dynamics.     

Photochemical crosslinking of bacteriophage T4 single-stranded DNA-binding protein (gp32) to oligo-p(dT)8: identification of phenylalanine-183 as the site of crosslinking

Using ultraviolet light, both the 33,000-dalton single-stranded DNA-binding protein from T4 bacteriophage (gp32) as well as a 25,000-dalton limited trypsin cleavage product of gp32 (core gp32*) that retains high affinity for single-stranded DNA can be crosslinked to an oligodeoxynucleotide, p(dT)8. After photolysis, a single tryptic peptide crosslinked to p(dT)8 was isolated by anion-exchange high-performance liquid chromatography. Gas-phase sequencing of this modified peptide gave the following sequence: Gln-Val-Ser-Gly-(X)-Ser-Asn-Tyr-Asp-Glu-Ser-Lys, which corresponds to residues 179-190 in gp32. Based on the absence of the expected phenylthiohydantoin derivative of phenylalanine 183 at cycle 5 (X) we infer that crosslinking has occurred at this position and that phenylalanine 183 is at the interface of the gp32:p(dT)8 complex in an orientation that allows covalent bond formation with the thymine radical produced by ultraviolet irradiation.     

Detection of proteins and phenol in DNA samples with second-derivative absorption spectroscopy.

We have employed near-uv second-derivative spectra of DNA, N-acetyl-L-tryptophanamide, N-acetyl-L-tyrosinamide, N-acetyl-L-phenylalanine ethyl ester, and phenol in a matrix least-squares multicomponent analysis algorithm to detect the presence of tryptophan, tyrosine, phenylalanine, and/or phenol in DNA preparations. With this method, each of these compounds can be detected in a DNA sample (absorbance, 0.1) at absorbance levels of less than 0.002. In practice, the presence of proteins can be detected at absorbance levels of less than 0.003. Using second-derivative spectra of proteins, contents of mixtures of proteins and DNA can be determined with less than 1% error. Mixtures of DNA and RNA can also be quantitatively analyzed with an error of approximately 2%. This technique can be easily implemented with computer-controlled spectrophotometers equipped with standard spectral analysis software. With prerecorded standard spectra, the time of analysis does not exceed a few seconds.     

Spin-labeling proton NMR study on aromatic amino acid residues in the guanine nucleotide binding site of human c-Ha-ras(1-171) protein

A truncated human c-Ha-ras gene product, ras(1-171) protein, was prepared and chemically modified with maleimide spin-label (MSL). By trypsin digestion of the MSL-labeled ras(1-171) protein, MSL-labeled peptide fragments were isolated and sequenced. The cysteine residue in position 118 of the protein, but not the other cysteine residues, Cys-51 or Cys-80, was found to be specifically labeled by MSL. The ESR spectrum of the MSL-labeled ras(1-171) protein indicates that the MSL group attached to Cys-118 is strongly immobilized. Proton NMR spectra at 400-MHz were measured for this MSL-labeled ras(1-171) protein and also for a control sample of a labeled ras(1-171) protein whose MSL was reduced by sodium ascorbate. In the difference spectra for these two proteins, resonances of protons in the vicinity of the MSL group attached to Cys-118 of the ras(1-171) protein were observed. Thus, the MSL group was found to be in the vicinity of the protein-bound GDP. A phenylalanine residue and two histidine residues, which were characterized by 2D HOHAHA and DQF-COSY spectra, were also found to be in the vicinity of MSL. NOE and pH titration analyses indicate that this phenylalanine residue is close to the bound GDP and one of the two histidine residues. By carboxypeptidase digestion, the two histidine residues near MSL were identified as His-27 and His-94.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactions controlling the membrane binding of basic protein domains: phenylalanine and the attachment of the myristoylated alanine-rich C-kinase substrate protein to interfaces.

Basic residues are known to play a critical role in the attachment of protein domains to membrane interfaces. Many of these domains also contain hydrophobic residues that may alter the binding and the position of the domain on the interface. In the present study, the role of phenylanine in determining the membrane position, dynamics and free energy of a peptide derived from the effector domain of the myristoylated alanine-rich C-kinase substrate (MARCKS) protein was examined. Deuterium NMR in membranes containing phosphatidylcholine (PC) and phosphatidylserine (PS) indicates that this peptide, MARCKS(151-175), partially penetrates the membrane interface when bound and alters the effective charge density on the membrane interface by approximately 2 charges per bound peptide. However, a derivative of this peptide in which the five phenylalanines are replaced by alanine, MARCKS-Ala, does not penetrate the interface when membrane-bound. This result was confirmed by depth measurements by electron paramagnetic resonance spectroscopy on several spin-labeled derivatives of the Phe-less derivative. In contrast to nitroxides on MARCKS(151-175), nitroxides on the derivative lacking Phe do not reside within the bilayer but are in the aqueous phase when the peptide is bound to the membrane. The Phe to Ala substitutions shift the position of the labeled side chains by approximately 10-15 A. The side-chain dynamics of MARCKS-Ala are strongly influenced by membrane charge density and indicate that this peptide is drawn closer to the membrane interface at higher charge densities. As expected, MARCKS-Ala binds more weakly to membranes composed of PS/PC (1:9) than does the native MARCKS peptide; however, each phenylalanine contributes only 0.2 kcal/mol to the binding energy difference, far less than the 1.3 kcal/mol expected for the binding of phenylalanine to the membrane interface. This energetic discrepancy and the differences in membrane position of these peptides can be accounted for by a dehydration energy that is encountered as the peptide approaches the membrane interface. This energy likely includes a Born repulsion acting between the charged peptide and the low dielectric membrane interior. The interplay between the long-range attractive Coulombic force, the short-range repulsive force and the hydrophobic effect controls the position and energetics of protein domains on acidic membrane interfaces.     

Structural role of the tyrosine residues of cytochrome c.

The tertiary structures of horse, tuna, Neurospora crassa, horse [Hse65,Leu67]- and horse [Hse65,Leu74]-cytochromes c were studied with high-resolution 1H n.m.r. spectroscopy. The amino acid sequences of these proteins differ at position 46, which is occupied by phenylalanine in the horse proteins but by tyrosine in the remaining two, and at positions 67, 74 and 97, which are all occupied by tyrosine residues in horse and tuna cytochrome c but in the other proteins are substituted by phenylalanine or leucine, though there is only one such substitution per protein. The various aromatic-amino-acid substitutions do not seriously affect the protein structure.     

Selenotyrosine and related phenylalanine derivatives.

A new series of Se-substituted phenylalanine derivatives has been synthesized having the para position of the phenyl ring substituted by selenocyanate (-SeCN), seleninic acid (-SeO(2)H), or selenol (-SeH) functional groups. The starting material for synthesis was 4'-aminophenylalanine, which is readily available in DL- or L- forms. Selenium was incorporated into the ring by reacting the unprotected amino acid with nitrous acid, followed by reaction of the diazotized aromatic amine with potassium selenocyanate at pH 4-5 to give phenylalanine selenocyanate. The selenocyanate derivative was converted to the selenol directly by reduction with sodium borohydride, or oxidized to the seleninic acid, which was then reduced to the selenol. Alkylation of the selenol ('selenotyrosine') gave the selenoether derivatives of phenylalanine [(Phe-SeR), R=methyl or allyl], and air oxidation of the selenol gave the diselenide. Mild oxidation of the selenoether 4'-(MeSe)Phe with peroxide gave the selenoxide derivative, 4'-[Se(O)Me]. Because of their stability and useful redox properties, aromatic selenoamino acids can be used as synthetic analogues to increase chemical functionality in proteins or peptides, and have potential pharmaceutical or nutritional applications. The possibility that aromatic selenoamino acids could be formed metabolically through reactions of reactive selenium intermediates with aromatic amino acid residues is discussed.     

Antisense suppression of proline degradation improves tolerance to freezing and salinity in Arabidopsis thaliana

The location of tryptophan residues in the actin macromolecule was studied on the basis of the known 3D structure. For every tryptophan residue the polarity and packing density of their microenvironments were evaluated. To estimate the accessibility of the tryptophan residues to the solvent molecules it was proposed to analyze the radial dependence of the packing density of atoms in the macromolecule about the geometric center of the indole rings of the tryptophan residues. The proposed analysis revealed that the microenvironment of tryptophan residues Trp-340 and Trp-356 has a very high density. So these residues can be regarded as internal and inaccessible to solvent molecules. Their microenvironment is mainly formed by non-polar groups of protein. Though the packing density of the Trp-86 microenvironment is lower, this tryptophan residue is apparently also inaccessible to solvent molecules, as it is located in the inner region of macromolecule. Tryptophan residue Trp-79 is external and accessible to the solvent. All residues that can affect tryptophan fluorescence were revealed. It was found that in the close vicinity of tryptophan residues Trp-79 and Trp-86 there are a number of sulfur atoms of cysteine and methionine residues that are known to be effective quenchers of tryptophan fluorescence. The most essential is the location of SG atom of Cys-10 near the NE1 atom of the indole ring of tryptophan residue Trp-86. On the basis of microenvironment analysis of these tryptophan residues and the evaluation of energy transfer between them it was concluded that the contribution of tryptophan residues Trp-79 and Trp-86 must be low. Intrinsic fluorescence of actin must be mainly determined by two other tryptophan residues--Trp-340 and Trp-356. It is possible that the unstrained conformation of tryptophan residue Trp-340 and the existence of aromatic rings of tyrosine and phenylalanine and proline residues in the microenvironments of tryptophan residues Trp-340 and Trp-356 are also essential to their blue fluorescence spectrum.     

Photoacoustic spectroscopy of aromatic amino acids in proteins

This paper concerns the use of photoacoustic spectroscopy (PAS) to study the presence of aromatic amino acid in proteins. We examined the aromatic amino acids in six proteins with well-known structures using absorption spectra of near ultraviolet PAS over the wavelength range 240–320 nm. The fundamental understanding of the physical and chemical properties that govern the absorption of light and a subsequent release of heat to generate a transient pressure wave was used to test the concept of monitoring aromatic amino acids with this method. Second derivative spectroscopy in the ultraviolet region of proteins was also used to study the regions surrounding the aromatics and the percentage area in each band was related in order to determine the contribution in function of the respective molar extinction coefficients for each residue. Further investigation was conducted into the interaction between sodium dodecyl sulphate (SDS) and bothropstoxin-I (BthTx-I), with the purpose of identifying the aromatics that participate in the interaction. The clear changes in the second derivative and curve-fitting procedures suggest that initial SDS binding to the tryptophan located in the dimer interface and above 10 SDS an increased intensity between 260 and 320 nm, demonstrating that the more widespread tyrosine and phenylalanine residues contribute to the SDS/BthTx-I interactions. These results demonstrate the potential of near UV-PAS for the investigation of membrane proteins/detergent complexes in which light scattering is significant.     

UV–Vis spectroscopy of tyrosine side-groups in studies of protein structure. Part 1: basic principles and properties of tyrosine chromophore

Spectroscopic properties of tyrosine residues may be employed in structural studies of proteins. Here we discuss several different types of UV–Vis spectroscopy, like normal, difference and second-derivative UV absorption spectroscopy, fluorescence spectroscopy, linear and circular dichroism spectroscopy, and Raman spectroscopy, and corresponding optical properties of the tyrosine chromophore, phenol, which are used to study protein structure.     

Quantitative determination of tryptophanyl and tyrosyl residues of proteins by second-derivative fluorescence spectroscopy

Second-derivative spectroscopy has been applied to the study of the fluorescence of aromatic amino acids. The spectral features of the second derivative emission spectra of free aromatic amino acids and proteins are described, the emission of each aromatic fluorophore being characterized by a particular minimum-maximum pair. An easy, accurate, and rapid method is proposed for the quantitative determination of tyrosine and tryptophan, based on the addition of small amounts of a standard solution to the samples followed by the measurement of the increase in the distance between a selected minimum and an adjacent maximum, in the second-derivative spectrum. For tyrosine determination, excitation wavelength was 275 nm, and the selected minimum-maximum (m,M) pair was (300; 330 nm), while an excitation of 300 nm and a minimum-maximum pair (357; 377 nm) were employed for the tryptophan determination. This method enables the tryptophan content of proteins to be determined directly, without the need for correction for the presence of tyrosine. The tyrosine content of proteins can also be determined at neutral pH, in the presence of both tryptophan and phenylalanine. The proposed method has also been applied to trypsin activation of frog epidermis tyrosinase.     

Protein and cDNA sequence of a glycine-rich, dimethylarginine-containing region located near the carboxyl-terminal end of nucleolin (C23 and 100 kDa)

By a combination of protein chemistry and recombinant DNA methods a glycine-rich region was found to be located near the carboxyl terminus of the nucleolar specific phosphoprotein, nucleolin, from Novikoff hepatoma (protein C23) and Chinese hamster ovary cells (100-kDa nucleolar protein). A sequence of 192 amino acid residues was derived from partial sequences of cyanogen bromide and N-bromosuccinimide fragments of protein C23 and deduced protein sequence from Chinese hamster ovary cell 100-kDa cDNA sequences. The 66 residues sequenced by protein methods were identical to the corresponding residues deduced by DNA sequencing. The multiple residues of NG,NG-dimethylarginine (DMA) contained in the nucleolin polypeptide were found to be limited to a segment of less than 10 kDa near the carboxyl-terminal end of the protein. This segment also contained internally repeated sequences (e.g. 7 copies of the sequence Gly-Gly-Arg-Gly-Gly were found) which were unrelated to sequences closer to the amino-terminal end. Most arginine residues in this region were surrounded by 2 or 3 glycine residues and were relatively close in sequence to phenylalanine residues.     

Effect of calcium binding on the quenching time of self-fluorescence of Ca-binding proteins

Lifetimes of phenylalanine, tyrosine and tryptophan self-fluorescence of three Ca2+-binding proteins (parvalbumins pI 4.47 and 3.95 and bovine alpha-lactalbumin) in the Ca2+-saturated state and without Ca2+ were measured on a device functioning in a channel of synchrotron radiation of the Lebedev Physical Institute electron accelerator C-60 with a single photon counting system. The decay curve of phenylalanine fluorescence of Ca2+-saturated parvalbumin pI 4.47 is two-exponential, which results from the presence of two subsystems of phenylalanine residues in this protein. Radiation of these subsystems is almost independent of one another. Detachment of Ca2+ from protein disturbs these subsystems. In case of tyrosine fluorescence of carp parvalbumin pI 3.95 a change in the quantum yield value of the stationary fluorescence induced by elimination of Ca2+ proceeds without a change of fluorescence lifetime. This seems to be related to the existence of static quenching of fluorescence in this case at the expense of complex formation between the chromophore and some adjacent quenching groups. Detachment of Ca2+ from alpha-lactalbumin induces conformational changes in its structure. The latter result in a transition of a number of tryptophane residues from its interior to the surface of the globule which is reflected in an increase of fluorescence quantum yield duration. It is concluded that in Ca2+-saturated alpha-lactalbumin some tryptophane residues are located near the quenching groups (dynamic quenching), most likely the disulfide bridges.     

Detection of methylated asparagine and glutamine residues in polypeptides

A residue of gamma-N-methylasparagine (gamma-NMA) is found at position beta-72 of many phycobiliproteins. delta-N-Methylglutamine is present in some bacterial ribosomal proteins. gamma-NMA was synthesized by reacting the omega-methyl ester of aspartate with methylamine and delta-N-methylglutamine by reaction of pyroglutamate with methylamine. These derivatives and the omega-methyl esters of aspartate and glutamate were characterized by melting point, by thin-layer chromatography, by amino acid analysis, by NMR spectroscopy, and after conversion to the phenylthiohydantoin (PTH) derivative. The gamma-NMA residues in peptides from allophycocyanin, C-phycocyanin, and B-phycoerythrin were stable under the conditions of automated sequential gas-liquid phase Edman degradation. On HPLC, PTH-gamma-NMA co-eluted with PTH-serine and was accompanied by a minor component eluting just prior to dimethylphenylthiourea. Similar results were obtained on manual derivatization of synthetic gamma-NMA to prepare the PTH derivative. The PTH-delta-N-methylglutamine standard eluted near the position of dimethylphenylthiourea under the usual conditions employed for the identification of PTH-amino acid derivatives in automated protein sequencing.     

Mistranslation in twelve Escherichia coli ribosomal proteins. Cysteine misincorporation at neutral amino acid residues other than tryptophan

The misincorporation of cysteine (codon: UGU/C) into twelve ribosomal proteins devoid of cysteine has been studied. Although it is generally assumed that cysteine is misincorporated at arginine and tryptophan residues (codons: CGU/U and UGG respectively), our results are consistent with the idea that cysteine is also misincorporated at phenylalanine residues (codon: UUU/C) through a second-position C:U mismatch. Cysteine was found in ribosomal proteins L29, L32/L33 and S10, under conditions where only its misincorporation at neutral residues was measured. Since these proteins contain no tryptophan, the date imply that cysteine has replaced a neutral amino acid other than tryptophan. Because there was a statistically significant correlation between the total level of cysteine in the twelve proteins under study and their content of phenylalanine and arginine residues, we conclude that there is a likelihood of cysteine misincorporation at phenylalanine residues, in addition to its misincorporation at arginine and tryptophan residues. Our measurements are consistent with the existence of a cluster of ribosomal proteins having an average mistranslation frequency of 2.5 X 10(-4)/residue and another having an average mistranslation frequency of 10(-3)/residue. There was three times less cysteine misincorporated into ribosomal protein L1 than into L7/L12, although the L1 mRNA contains eleven CGU/C codons and four UUU/C codons while the L7/L12 mRNA contains only one arginine and two phenylalanine codons (both proteins are free of tryptophan). Furthermore, the mRNAs for both L1 and L7/L12 contain a CGU codon located in the context GUA-codon-GG and there was as much cysteine incorporated at this codon in L7/L12 [Bouadloun, F., Donner, D. and Kurland, C.G. (1983) EMBO J. 2, 1351-1356] than in the whole of L1. This suggests that, relatively speaking, little cysteine is to be found at the phenylalanine and the other ten arginine positions of L1 and that the phenylalanine residues of L7/L12 are particularly error-prone.     

Engineered oligomerization state of OmpF protein through computational design decouples oligomer dissociation from unfolding

Biogenesis of β-barrel membrane proteins is a complex, multistep, and as yet incompletely characterized process. The bacterial porin family is perhaps the best-studied protein family among β-barrel membrane proteins that allows diffusion of small solutes across the bacterial outer membrane. In this study, we have identified residues that contribute significantly to the protein-protein interaction (PPI) interface between the chains of outer membrane protein F (OmpF), a trimeric porin, using an empirical energy function in conjunction with an evolutionary analysis. By replacing these residues through site-directed mutagenesis either with energetically favorable residues or substitutions that do not occur in natural bacterial outer membrane proteins, we succeeded in engineering OmpF mutants with dimeric and monomeric oligomerization states instead of a trimeric oligomerization state. Moreover, our results suggest that the oligomerization of OmpF proceeds through a series of interactions involving two distinct regions of the extensive PPI interface: two monomers interact to form a dimer through the PPI interface near G19. This dimer then interacts with another monomer through the PPI interface near G135 to form a trimer. We have found that perturbing the PPI interface near G19 results in the formation of the monomeric OmpF only. Thermal denaturation of the designed dimeric OmpF mutant suggests that oligomer dissociation can be separated from the process of protein unfolding. Furthermore, the conserved site near G57 and G59 is important for the PPI interface and might provide the essential scaffold for PPIs.     

Analysis of the murine ecotropic leukemia virus receptor reveals a common biochemical determinant on diverse cell surface receptors that is essential to retrovirus entry.

Two residues, tyrosine 235 and glutamic acid 237, of the ecotropic murine leukemia virus receptor (ATRC1) have been shown to be essential for receptor-mediated virus envelope binding and entry. We performed genetic analyses to examine the biochemical contribution of these residues in a productive virus-receptor interaction. Altered ATRC1 receptors bearing either a phenylalanine, a tryptophan, a histidine, or a methionine at position 235 mediated ecotropic virus entry comparable to that mediated by ATRC1. In contrast, altered ATRC1 receptors bearing alanine, threonine, serine, or proline at position 235 exhibited a 300- to 10,000-fold decrease in receptor capability. Furthermore, substitution of tyrosine or phenylalanine into the corresponding position (242) of the homologous human protein that lacks ecotropic virus receptor capability resulted in acquisition of ecotropic virus receptor function comparable to that of ATRC1. Substitution of a tryptophan or a histidine at that position of the human protein, however, resulted in a much-reduced receptor capability, suggesting a preference for a benzene ring in the hydrophobic side chain. A similar analysis of proteins substituted at position 237 revealed that aspartic acid, but not arginine or lysine, can functionally substitute for glutamic acid 237 in ATRC1 or at the corresponding position in the human protein. These results suggest a requirement for an acidic and a nearby hydrophobic amino acid for efficient ecotropic virus entry. Similar motifs have been identified in the virus binding sites of other retrovirus receptors, suggesting that the initial step of retrovirus entry may be governed by a common mechanism.